Recombinant DNA vaccines protect against tumors that are resistant to recombinant vaccinia vaccines containing the same gene

GITR ligand fusion protein agonist enhances the tumor antigen-specific CD8 T-cell response and leads to long-lasting memory

BACKGROUND

The expansion of antigen-specific CD8 T cells is important in generating an effective and long-lasting immune response to tumors and viruses. Glucocorticoid-induced tumor necrosis factor receptor family-related receptor (GITR) is a co-stimulatory receptor that binds the GITR ligand (GITRL). Agonism of GITR can produce important signals that drive expansion of effector T cell populations.

METHODS

We explored two separate murine tumor models, CT26 and TC-1, for responsiveness to GITR Ligand Fusion Protein(GITRL-FP) monotherapy. In TC-1, GITRL-FP was also combined with concurrent administration of an E7-SLP vaccine. We evaluated tumor growth inhibition by tumor volume measurements as well as changes in CD8 T cell populations and function including cytokine production using flow cytometry. Additionally, we interrogated how these therapies resulted in tumor antigen-specific responses using MHC-I dextramer staining and antigen-specific restimulations.

RESULTS

In this study, we demonstrate that a GITR ligand fusion protein (GITRL-FP) is an effective modulator of antigen-specific CD8 T cells. In a CT26 mouse tumor model, GITRL-FP promoted expansion of antigen-specific T cells, depletion of regulatory T cells (Tregs), and generation of long-lasting CD8 T cell memory. This memory expansion was dependent on the dose of GITRL-FP and resulted in complete tumor clearance and protection from tumor rechallenge. In contrast, in TC-1 tumor-bearing mice, GITRL-FP monotherapy could not prime an antigen-specific CD8 T cell response and was unable to deplete Tregs. However, when combined with a vaccine targeting E7, treatment with GITRL-FP resulted in an augmentation of the vaccine-induced antigen-specific CD8 T cells, the depletion of Tregs, and a potent antitumor immune response. In both model systems, GITR levels on antigen-specific CD8 T cells were higher than on all other CD8 T cells, and GITRL-FP interacted directly with primed antigen-specific CD8 T cells.

CONCLUSIONS

When taken together, our results demonstrate that the delivery of GITRL-FP as a therapeutic can promote anti-tumor responses in the presence of tumor-specific CD8 T cells. These findings support further study into combination partners for GITRL-FP that may augment CD8 T-cell priming as well as provide hypotheses that can be tested in human clinical trials exploring GITR agonists including GITRL-FP.

A prime/boost strategy using DNA/fowlpox recombinants expressing the genetically attenuated E6 protein as a putative vaccine against HPV-16-associated cancers

BACKGROUND

Considering the high number of new cases of cervical cancer each year that are caused by human papilloma viruses (HPVs), the development of an effective vaccine for prevention and therapy of HPV-associated cancers, and in particular against the high-risk HPV-16 genotype, remains a priority. Vaccines expressing the E6 and E7 proteins that are detectable in all HPV-positive pre-cancerous and cancer cells might support the treatment of HPV-related lesions and clear already established tumors.

METHODS

In this study, DNA and fowlpox virus recombinants expressing the E6F47R mutant of the HPV-16 E6 oncoprotein were generated, and their correct expression verified by RT-PCR, Western blotting and immunofluorescence. Immunization protocols were tested in a preventive or therapeutic pre-clinical mouse model of HPV-16 tumorigenicity using heterologous (DNA/FP) or homologous (DNA/DNA and FP/FP) prime/boost regimens. The immune responses and therapeutic efficacy were evaluated by ELISA, ELISPOT assays, and challenge with TC-1* cells.

RESULTS

In the preventive protocol, while an anti-E6-specific humoral response was just detectable, a specific CD8(+) cytotoxic T-cell response was elicited in immunized mice. After the challenge, there was a delay in cancer appearance and a significant reduction of tumor volume in the two groups of E6-immunized mice, thus confirming the pivotal role of the CD8(+) T-cell response in the control of tumor growth in the absence of E6-specific antibodies. In the therapeutic protocol, in-vivo experiments resulted in a higher number of tumor-free mice after the homologous DNA/DNA or heterologous DNA/FP immunization.

CONCLUSIONS

These data establish a preliminary indication for the prevention and treatment of HPV-related tumors by the use of DNA and avipox constructs as safe and effective immunogens following a prime/boost strategy. The combined use of recombinants expressing both E6 and E7 proteins might improve the antitumor efficacy, and should represent an important approach to control HPV-associated cancers.

Cancer vaccination drives Nanog-dependent evolution of tumor cells toward an immune-resistant and stem-like phenotype

Due to the exquisite specificity and potency of the immune system, vaccination is in theory the most precise and powerful approach for controlling cancer. However, current data from clinical trials indicate that vaccination rarely yields significant benefits for cancer patients in terms of tumor progression and long-term survival. The poor clinical outcomes of vaccination are primarily caused by mechanisms of immune tolerance, especially within the tumor microenvironment. Here, we report that vaccination drives the evolution of tumor cells toward an immune-resistant and stem-like phenotype that promotes tumor growth and nullifies the CTL response. The emergence of this phenotype required the transcription factor Nanog, which is induced as a consequence of immune selection. Nanog expression enhanced the stem-like features of tumor cells and protected them from killing by tumor-reactive CTLs. Delivery of siNanog into tumor-bearing mice rendered the tumor vulnerable to immune surveillance and strongly suppressed its growth. Together, our findings show tumor adaptation to vaccination through gain of an immune-resistant, stem-like phenotype and identify Nanog as a central molecular target in this process. Future vaccination technology should consider Nanog an important target to enhance the immunotherapeutic response.

Fowlpox virus recombinants expressing HPV-16 E6 and E7 oncogenes for the therapy of cervical carcinoma elicit humoral and cell-mediated responses in rabbits

Background

Around half million new cases of cervical cancer arise each year, making the development of an effective therapeutic vaccine against HPV a high priority. As the E6 and E7 oncoproteins are expressed in all HPV-16 tumour cells, vaccines expressing these proteins might clear an already established tumour and support the treatment of HPV-related precancerous lesions.

Methods

Three different immunisation regimens were tested in a pre-clinical trial in rabbits to evaluate the humoral and cell-mediated responses of a putative HPV-16 vaccine. Fowlpoxvirus (FP) recombinants separately expressing the HPV-16 E6 (FP_E6) and E7 (FP_E7) transgenes were used for priming, followed by E7 protein boosting.

Results

All of the protocols were effective in eliciting a high antibody response. This was also confirmed by interleukin-4 production, which increased after simultaneous priming with both FP_E6 and FP_E7 and after E7 protein boost. A cell-mediated immune response was also detected in most of the animals.

Conclusion

These results establish a preliminary profile for the therapy with the combined use of avipox recombinants, which may represent safer immunogens than vaccinia-based vectors in immuno-compromised individuals, as they express the transgenes in most mammalian cells in the absence of a productive replication.

Enhancing the therapeutic effect against ovarian cancer through a combination of viral oncolysis and antigen-specific immunotherapy

Cancer therapy using oncolytic viruses represents a promising new approach for controlling ovarian cancer. In this study, we have circumvented the limitation of repeated vaccination by employing different virus vectors, Semliki Forest Virus (SFV) and vaccinia virus (VV) for boosting the immune response. We found that infection of tumor-bearing mice with VV followed by infection with SFV or vice versa leads to enhanced antitumor effects against murine ovarian surface epithelial carcinoma (MOSEC) tumors. Furthermore, infection with VV-ovalbumin (OVA) followed by infection with SFV-OVA or vice versa was found to lead to enhanced OVA-specific CD8(+) T-cell immune responses. In addition, we found that infection with SFV-OVA followed by infection with VV-OVA leads to enhanced antitumor effects in vivo and enhanced tumor killing in vitro through a combination of viral oncolysis and antigen-specific immunity. The clinical implications of this study are discussed.

Phase I Study of a Plasmid DNA Vaccine Encoding MART-1 in Patients with Resected Melanoma at Risk for Relapse

Immunization with plasmid DNA represents an attractive method for increasing cellular immune responses against cancer antigens. The safety and immunologic response of a plasmid encoding the MART-1 melanocyte differentiation antigen was evaluated in 12 patients with resected melanoma at risk for relapse. As a control, patients were also administered a plasmid encoding hepatitis B surface antigen (HBsAg). After establishing immunologic activity of the vaccines in mice, groups of three to six HLA-A2-positive patients were enrolled into one of three cohorts in which they received intramuscular injections of the MART-1 plasmid into the right deltoid and the HBsAg plasmid into the left deltoid at doses of 0.1, 0.3, or 1.0 mg on days 1, 43, 85, and 127. Injections were well tolerated. Toxicity was limited to grade 1 pain and injection site tenderness. Systemic toxicity was not observed. Although baseline MART-1-specific lymphoproliferative and ELISPOT responses were evident, no patient manifested increases after injection of the MART-1 plasmid. Furthermore, changes in MART-1-specific precursors were not evident after immunization as assessed by an in vitro stimulation assay. No patients manifested a lymphoproliferative response to HBsAg antigen, and significant antibody responses to HBsAg were also not observed. Although injections were safe, the authors could not show significant immunologic responses to plasmid encoding MART-1 or HBsAg using the dose, schedule, and route of administration applied. This study underscores species differences in the ability to respond to plasmid immunogens.

DNA vaccination against tumors

DNA vaccines have been used to generate protective immunity against tumors in a variety of experimental models. The favorite target antigens have been those that are frequently expressed by human tumors, such as carcinoembryonic antigen (CEA), ErbB2/neu, and melanoma-associated antigens. DNA vaccines have the advantage of being simple to construct, produce and deliver. They can activate all arms of the immune system, and allow substantial flexibility in modifying the type of immune response generated through codelivery of cytokine genes. DNA vaccines can be applied by intramuscular, dermal/epidermal, oral, respiratory and other routes, and pose relatively few safety concerns. Compared to other nucleic acid vectors, they are usually devoid of viral or bacterial antigens and can be designed to deliver only the target tumor antigen(s). This is likely to be important when priming a response against weak tumor antigens. DNA vaccines have been more effective in rodents than in larger mammals or humans. However, a large number of methods that might be applied clinically have been shown to ameliorate these vaccines. This includes in vivo electroporation, and/or inclusion of various immunostimulatory molecules, xenoantigens (or their epitopes), antigen-cytokine fusion genes, agents that improve antigen uptake or presentation, and molecules that activate innate immunity mechanisms. In addition, CpG motifs carried by plasmids can overcome the negative effects of regulatory T cells. There have been few studies in humans, but recent clinical trials suggest that plasmid/virus, or plasmid/antigen-adjuvant, prime-boost strategies generate strong immune responses, and confirm the usefulness of plasmid-based vaccination.

Expression library immunization to discover and improve vaccine antigens

Genetic immunization is a novel method for vaccination in which DNA is delivered into the host to drive both cellular and humoral immune responses against its protein product. While genetic immunization can be potent, it requires that one have, in hand, a gene that encodes a protective protein antigen. Therefore, for many diseases, one cannot make a genetic vaccine because no protective antigen is known or no gene for this antigen is available. This lack of candidate antigens and their genes is a considerable bottleneck in developing new vaccines against old infectious agents, new emerging pathogens, and bioweapons. To address this limitation, we developed expression library immunization (ELI) as a high-throughput technology to discover vaccine candidate genes at will, by using the immune system to screen the entire genome of a pathogen for vaccine candidate. To date, ELI has discovered new vaccine candidates from a diverse set of bacterial, fungal, and parasitic pathogens. In addition, the process of applying ELI to the genome of pathogens allows one to genetically re-engineer these antigens to convert immunoevasive pathogen proteins into immunostimulatory vaccine antigens. Therefore, ELI is a potent technology to discover new vaccines and also generate genomic vaccines with amplified, multivalent immunostimulatory capacities.

CD8+ T cells, NK cells and IFN-gamma are important for control of tumor with downregulated MHC class I expression by DNA vaccination

One of the major hurdles facing cancer immunotherapy is that cancers may downregulate expression of MHC class I molecules. The development of a suitable tumor model with downregulated MHC class I expression is critical for designing vaccines and immunotherapeutic strategies to control such tumors. We developed an E7-expressing murine tumor model with downregulated MHC class I expression, TC-1 P3 (A15). Using this model, we tested DNA and vaccinia vaccines for their ability to control tumors with downregulated MHC class I expression. We found that vaccination with DNA encoding E7 linked to Mycobacterial heat shock protein 70 (HSP70) generated a significant antitumor effect against TC-1 P3 (A15), while vaccination with E7/HSP70 vaccinia did not generate an appreciable antitumor effect. Lymphocyte depletion experiments revealed that both CD8+ T cells and NK cells were essential for the antitumor effect generated by E7/HSP70 DNA against TC-1 P3 (A15). Furthermore, tumor protection experiments using IFN-gamma knockout mice revealed that IFN-gamma was essential for the antitumor effect generated by E7/HSP70 DNA against TC-1 P3 (A15). Our results demonstrate that vaccination with E7/HSP70 DNA results in a significant antitumor effect against a neoplasm with downregulated MHC class I expression and the importance of CD8+ T cells, NK cells, and IFN-gamma in generating this antitumor effect.

Immunisation with modified HPV16 E7 genes against mouse oncogenic TC-1 cell sublines with downregulated expression of MHC class I molecules

Human papillomavirus type 16 (HPV16)-transformed mouse TC-1 cells are extensively used in the evaluation of efficacy of experimental vaccines against tumours induced by HPVs. As these cells strongly express MHC class I molecules and downregulation of MHC class I surface expression is one of the important mechanisms that enable tumour escape from the host immune system, we undertook to derive TC-1 clones with reduced expression of MHC class I antigens. TC-1 cells were inoculated into mice preimmunised with an E7 gene-based DNA vaccine and from tumours developing in a portion of the animals, cell clones with downregulated MHC class I surface expression were isolated. Treatment with IFN-gamma resulted in an upregulation of MHC class I molecules in these cells, but after IFN-gamma removal, their expression gradually dropped again. When the expression of some components of the antigen-processing machinery (APM; LMP-2, TAP-1, and TAP-2) was tested, a reduced TAP-1 production was detected in cell lines with downregulated MHC class I expression. An enhanced immunoresistance of TC-1-derived clones with reduced MHC class I expression was observed in animals immunised with plasmids carrying modified E7 genes. Apart from the previously described fusion gene Sig/E7/LAMP-1, a new construct, Sig/E7GGG/LAMP-1, with a mutated Rb-binding site, was also used for immunisation. No significant change of immunogenicity was recorded for Sig/E7GGG/LAMP-1. Cell lines with downregulated MHC class I expression derived from TC-1 cells may represent a useful model for testing therapeutic anti-HPV vaccines in settings more relevant to clinical requirements.

The potential of DNA vaccination against tumor-associated antigens for antitumor therapy

Conventional treatment approaches for malignant tumors are highly invasive and sometimes have only a palliative effect. Therefore, there is an increasing demand to develop novel, more efficient treatment options. Increased efforts have been made to apply immunomodulatory strategies in antitumor treatment. In recent years, immunizations with naked plasmid DNA encoding tumor-associated antigens have revealed a number of advantages. By DNA vaccination, antigen-specific cellular as well as humoral immune responses can be generated. The induction of specific immune responses directed against antigens expressed in tumor cells and displayed e.g., by MHC class I complexes can inhibit tumor growth and lead to tumor rejection. The improvement of vaccine efficacy has become a critical goal in the development of DNA vaccination as antitumor therapy. The use of different DNA delivery techniques and coadministration of adjuvants including cytokine genes may influence the pattern of specific immune responses induced. This brief review describes recent developments to optimize DNA vaccination against tumor-associated antigens. The prerequisite for a successful antitumor vaccination is breaking tolerance to tumor-associated antigens, which represent "self-antigens." Currently, immunization with xenogeneic DNA to induce immune responses against self-molecules is under intensive investigation. Tumor cells can develop immune escape mechanisms by generation of antigen loss variants, therefore, it may be necessary that DNA vaccines contain more than one tumor antigen. Polyimmunization with a mixture of tumor-associated antigen genes may have a synergistic effect in tumor treatment. The identification of tumor antigens that may serve as targets for DNA immunization has proceeded rapidly. Preclinical studies in animal models are promising that DNA immunization is a potent strategy for mediating antitumor effects in vivo. Thus, DNA vaccines may offer a novel treatment for tumor patients. DNA vaccines may also be useful in the prevention of tumors with genetic predisposition. By DNA vaccination preventing infections, the development of viral-induced tumors may be avoided.